The present invention relates to a method and apparatus for separating mixtures of organic and aqueous liquid phases, and in particular to the separation of such mixtures into a substantially organic-free aqueous phase and a substantially aqueous-free organic phase.
The need for separation of organic from aqueous liquid phases is exemplified by fuel spills on bodies of water and soils, and the requisite clean-up activity. Various methods and apparatus have been used to contain and/or clean up the spill, to prevent damage to the environment. The need for separation also arises in the areas of water wetted fuels at storage terminals. As well, the removal of water from crude oil is a separation of prime concern to that industry.
One prior art approach has been to separate the organic from the aqueous phase using microporous hollow fibres, wherein a mixture of such materials is forced into contact with the inside surface (lumen) of the fibres, commonly referred to as the xe2x80x9cdownborexe2x80x9d approach. An example of this approach is in Nohmi et al. U.S. Pat. No. 4,229,297, which teaches a method of separating oil from an oil-containing liquid. It is well-known in the art that for a two-phase down-bore feed, the pressure drop over a given length of fibre is not predictable. Accordingly, Nohmi et al. would not be useful for separating such a mixture into water-free oil and oil-free water. That is, when the pressure exceeds the pressure at which both oil and water pass through the micropores ie. the break-through pressure of water, it is no longer possible to achieve the desired result.
Another approach is described in published Canadian application of G. Sutherland and C. Glassford, serial no. 2,248,280. In this approach, the feed stream is fed to the fibres from the xe2x80x9coutside-inxe2x80x9d. This is an improvement over a xe2x80x9cdownborexe2x80x9d feed since a much lower pressure drop is involved, permitting higher feedstream rates. Specifically, a method and apparatus are described, for separating an immiscible organic compound from an aqueous mixture containing the compound and an aqueous phase, the method comprising the steps of:
providing a plurality of hollow hydrophobic fibres having micropores therein and having exposed ends inaccessible to the mixture, the micropores extending from the outside of each fibre to the hollow interior thereof;
creating a pressure differential between the mixture and the hollow fibres such that the mixture is under higher pressure relative to the fibre lumen, the pressure being sufficient to permit passage of the organic compound but insufficient to allow aqueous passage into the micropores and for the hollow fibres to collapse;
contacting the micropores of the fibres with the mixture;
collecting the immiscible organic compound; and discharging the aqueous phase substantially devoid of the immiscible organic compound.
The apparatus employed by thr reference comprises a plurality of hollow hydrophobic fibres having micropores therein. The micropores extend through the fibres from the outside to the hollow interior or lumen. The pores sizes will vary, depending upon the material of which the fibre is made and the intended use of the fibre in terms of the organic contaminent to be removed. Generally, the pore size is large enough to permit an acceptable flux, but small enough to exclude water. Pore size ranges of 0.03 microns to about 5 microns are disclosed. A pressure differential as between the mixture and the fibre lumen is provided by a positive pressure pump.
It has been found that the method and apparatus of Sutherland and Glassford have certain drawbacks. First, there is no appreciation in the reference that the use of most positive displacement pumps in providing the pressurised feed stream to the fibres will cause the formation of xe2x80x98oil in waterxe2x80x99 emulsions. In such emulsions, organic liquid droplets become surrounded by water droplets, and thus not xe2x80x98recognisedxe2x80x99 by the fibre as organic phase. Such droplets will not pass through the micropores, leaving substantial amounts of organic liquid in the aqueous phase ie. the aqueous phase removed is not organic free. xe2x80x98Water-in-oilxe2x80x99 emulsions do not appear to be a problem, as the surrounding oil droplets are recognized by the fibre as organic, and the central water droplet is typically far larger than the pore size of the fibre.
Also, the reference does not recognise that certain organic phases, e.g. crude oil, contain large particle size, high molecular weight organic particulates such as waxes or asphaltines. These particulates can be of a size that greater than the pore diameter of the micropores. Being organic in nature, these particulates will tend to xe2x80x98blindxe2x80x99 the micropores, reducing and ultimately preventing the passage of the lower molecular weight oil.
Further, we have found that the Sutherland/Glassford technology, having two product streams, an aqueous product stream and an organic product stream, has production limitations. The restriction to two such product streams in itself limits productivity. Further, it was not recognized that the buildup of productivity reducing agents such as surfactants, also reduces productivity. More specifically, the reference does not recognise that most organic liquids such as crude oil, fuels, etc. will contain both natural and/or man-made surfactants. Man-made surfactants are frequently present in the form of scale inhibitors, corrosion inhibitors, oxygen scavengers and biocides. Such molecules, due to their xe2x80x98dual naturexe2x80x99, (having both aqueous and organic xe2x80x98portionsxe2x80x99) can build up on the fibre surface and occlude the pores. The method of operation taught in the reference can exasperate this build up, resulting in a decline in organic recovery rates and therefore overall productivity. It can also result in frequent shutdowns and affects continuous operation.
According to one aspect of the invention, a method is provided for separation mixtures containing an aqueous liquid phase and an immiscible organic phase, into a substantially organic-free aqueous phase and a substantially aqueous-free organic phase, comprising
a) providing a plurality of hollow hydrophobic fibres having micropores therein and having exposed ends inaccessible to the mixture, the micropores extending from an outside surface of each fibre to a hollow interior portion thereof,
b) pressurising the mixture in a controlled low shear manner to minimise emulsification of the organic and aqueous phases, such that the mixture is under higher pressure relative to the hollow interior of the fibres to provide a pressure differential between the mixture and the hollow interior of the fibres,
c) contacting the fibres with the pressurised mixture, the pressure differential being sufficient to permit passage through the micropores of the immiscible organic phase, but insuffucient to allow passage through the micropores of the aqueous phase, and for the fibres to collapse,
d) collecting the substantially organic-free aqueous phase which has not passed through the pores of the fibres, and
e) collecting the substantially free aqueous free organic phase which has passed through the pores of the fibres.
According to another aspect of the invention a method is provided for separating mixtures containing an aqueous phase and an immiscible organic phase which contains large organic particulates, into a substantially organic-free aqueous phase and a substantially aqueous-free organic phase, comprising
a) providing a plurality of hollow hydrophobic fibres having micropores therein and having exposed ends inaccessible to the mixture, the micropores extending from an outside surface of each fibre to a hollow interior portion thereof,
b) treating the mixture to reduce the particle size of the organic particulates to a size equal to or less than the pore size of the fibre,
c) pressurising the mixture in a controlled low shear manner to minimise emulsification of the organic and aqueous phases, such that the mixture is under higher pressure relative to the hollow interior of the fibres to provide a pressure differential between the mixture and the hollow interior of the fibres,
d) contacting the fibres with the pressurised mixture, the pressure differential being sufficient to permit passage through the micropores of the immiscible organic phase, but insuffucient to allow passage through the micropores of the aqueous phase, and for the fibres to collapse,
e) collecting the substantially organic-free aqueous phase which has not passed through the micropores, and
f) collecting the substantially free aqueous-free organic phase which has passed through the micropores.
According to yet another aspect of the invention, a method is provided for separating mixtures containing an aqueous liquid and an immiscible organic phase, such a mixture may also contain a surfactant or surfactants determined to be detrimental to the long term productivity of microporous hollow fibres, comprising,
a) providing a plurality of hollow hydrophobic fibres having micropores therein and having exposed ends inaccessible to the mixture, the micropores extending from an outside surface of each fibre to a hollow interior portion thereof,
b) pressurising the mixture in a controlled low shear manner to minimise emulsification of the organic and aqueous phases, such that the mixture is under higher pressure relative to the hollow interior of the fibres to provide a pressure differential between the mixture and the hollow interior of the fibres,
c) contacting the fibres with the pressurised mixture, the pressure differential being sufficient to permit passage through the micropores of the immiscible organic phase, but insuffucient to allow passage through the micropores of the aqueous phase, and for the fibres to collapse,
d) collecting as a first product stream which has not passed through the micropores, a substantially organic-free aqueous phase,
e) collecting as a second product stream which has passed through the micropores, a substantially aqueous-free organic phase, and
f) collecting as a third product stream, which has not passed through the micropores, a stream comprising an organic phase and  less than 0.5% of water.
As will be explained more fully, in the Detailed Decription of the inventioin that follows, those skilled in the art will recognise that this third product stream is the same quality of stream that can be obtained from a properly designed and operated coalescing device.
An advantage of this aspect of the current invention operated in this manner is the fact that the plurality of fibres can be housed within the same type of housing, thereby allowing three useful product streams to be produced from a single feed mixture. The amount of water in the third product stream is such that either this stream alone still xe2x80x98meets specxe2x80x99 and is therefore of useful quality, or in combination with some or all of the substantially aqueous-free organic phase, the resulting mixture still xe2x80x98meets specxe2x80x99 and is therefore of useful quality. The mixing of the third product stream with the substantially aqueous-free organic stream thus improves productivity.
According to a further aspect of the invention, an apparatus is provided for separating mixtures containing an aqueous liquid and an immiscible organic phase, such a mixture may also contain a surfactant or surfactants determined to be detrimental to the long term productivity of microporous hollow fibres, comprising,
a) microporous hydrophobic hollow fibre means, the fibres having micropores therein and having exposed ends inaccessible to the mixture, the micropores extending from an outside surface of each fibre to a hollow interior portion thereof,
b) means for pressurising the mixture in a controlled low shear manner to minimise emulsification of the organic and aqueous phases to provide a feed steam which is under higher pressure relative to the hollow interior of the fibres to provide a pressure differential between the mixture and the hollow interior of the fibres, such that when the fibres are contacted with the pressurised mixture, the pressure differential being sufficient to permit passage through the micropores of the immiscible organic phase, but insuffucient to allow passage through the micropores of the aqueous phase, and for the fibres to collapse,
c) means for collecting as a first product stream which has not passed through the micropores, a substantially organic-free aqueous phase, and
d) means for collecting as a second product stream which has passed through the micropores, a substantially aqueous-free organic phase.
In some embodiments, means is provided for collecting as a third product stream, which has not passed through the micropores, an organic phase containing  less than 0.5% of an aqueous liquid.
In some embodiments, means for controlling product flow is also provided in association with the third product stream.
The hollow fibres may be arranged in various manners, such as in an array, a diverging pattern, a parallel pattern, an intersecting pattern, bundled into one or more modules, incorporated into a mat structure etc. as will be apparent to those skilled in the art.
The fibre material is selected such that the contaminant and the processing conditions do not deleteriously affect the properties and structure of the fibres, with respect to lumen size, internal diameter, external diameter, pore size or surface characteristics, such as hydrophobicity. For example, the fibres do not collapse or exhibit any appreciable reduction in pore size in the presence of various organic contaminants, and will withstand the high operating temperatures of up to 70xc2x0 C. used in the processing of some materials. For most applications a temperature range of 30-70xc2x0 C. is appropriate. In some cases, such as involving heavy oils or complex mixtures of oils, the operating temperatures may be even higher.
For some types of fibres, their tendency to incur dimensional changes in the presence of such contaminants and high temperatures, is overcome by providing a stabilizing means e.g in the form of a tubesheet formed of a synthetic resin such as an epoxy resin. The stabilizing means maintains the relative positioning of the fibres in a spaced and connected relation to provide maximum fibre surface area exposure, and prevents kinking or excess distortion of the fibres, particularly when grouped in a bundle.
The pore size of the fibres will vary, depending upon the intended use of the fibres. Generally, the pore diameters will be large enough to permit an acceptable flux, but small enough to exclude water due to surface tension effects of the hydrophobic fibre.